UV light-emitting diodes as a radiation source in a device for the artificial weathering of samples

ABSTRACT

The usual xenon radiation source in conventional weathering testers is replaced by UV light-emitting diodes (LEDs) ( 6 ). These can be used to provide a good approximation of the UV component of the solar spectrum, in particular when different types of UV light-emitting diodes ( 6 ) with different emission characteristics are employed. Optionally, further light-emitting diodes in the visible spectral range may also be used in order to cover other parts of the solar spectrum. The light-emitting diodes ( 6 ) may be mounted on a flexible printed circuit board ( 5 ), which may in turn be fitted on a tubular holding body ( 4 ). The flexible printed circuit board carrying the light-emitting diodes may also be placed on other substrates ( 7 ) which are pliable in a geometrically stable way, so that that they can be arranged at a uniform distance from material samples with a particular surface topology, in order to obtain uniform exposure.

The present invention relates to a device for the artificial weatheringof samples according to the precharacterizing clause of Patent Claim 1.The weathering-dependent ageing of a sample, in particular of a flatmaterial sample, is evaluated in such devices by exposing the sample toartificial weathering. To that end, such devices usually have aweathering chamber which contains holding means for holding samples tobe weathered and a radiation source for applying radiation to thesamples, in particular UV radiation.

Such devices for the artificial weathering of samples are intended toestimate the lifetime of materials which are constantly exposed tonatural weather conditions during their use, and which therefore sufferfrom climatic effects such as sunlight, solar heat, moisture and thelike. In order to obtain a good simulation of the natural weatheringsituation, the spectral energy distribution of the light generated inthe device should correspond as closely as possible to that of naturalsolar radiation, for which reason xenon radiators are used as radiationsources in such devices. An accelerated ageing test of the materials isessentially achieved by much more intense irradiation of the samplescompared with natural conditions, which speeds up the ageing of thesamples. In this way, a prediction of the long-term ageing of a materialsample can be made after a comparatively short time.

A large number of the samples studied in artificial weathering devicesconsist of polymeric materials. Their deterioration due to weathering isessentially caused by the UV component of solar radiation. The primaryphotochemical processes which take place during this, that is to say theabsorption of photons and the generation of excited states or freeradicals, are independent of temperature. The subsequent reaction stepswith the polymers or additives, however, may be temperature-dependent sothat the observed ageing of the materials is also temperature-dependent.

A xenon lamp is normally used as the radiation source in the weatheringtesters of the prior art. Although such a lamp is known to be able tosimulate the solar spectrum very well, the emitted radiationnevertheless has a relatively high spectral component in the infraredspectral range, which needs to be suppressed by filters in order toprevent excessive heating of the samples. Furthermore, a commerciallyavailable xenon radiation source only has a lifetime of about 1500hours.

A halogen lamp may also be used as the radiation source, although thishas the disadvantage that it is not adjustable, or can only be adjustedto a minor extent. The same applies to fluorescent lamps, which likewisehave already been used as radiation sources in weathering testers andwhich also have the disadvantage of a relatively short lifetime.

All of the aforementioned radiation sources furthermore have thedisadvantage that they are not spectrally modifiable. But it is oftenuseful to study the ageing of a material sample as a function ofradiation in a limited wavelength range. For this purpose, admittedly,it is known to resolve the radiation of a xenon lamp into its spectralcomponents by using a prism or grating before sending it onto thesample: the different parts of the sample are then exposed to radiationwith a different wavelength and the property changes at different pointsof the sample can be unequivocally assigned to the wavelength of theincident radiation. This, however, needs relatively long exposure timessince the spectral exposure level on the sample itself is relativelylow.

Another disadvantage of the aforementioned conventional radiationsources in weathering testers, owing the way in which they are designedand operated, is that they are relatively unwieldy and, for example,modified conditions in respect of the sample surfaces of the materialsamples to be exposed cannot therefore be accommodated.

It is therefore an object of the present invention to provide a devicefor the artificial weathering of samples, in which the spectral andspatial features of the radiation source and the radiation emitted by itcan be adapted more flexibly and, in particular, modified situations ofthe samples to be exposed can be accommodated better.

This object is achieved by the characterizing features of Patent Claim1. The dependent claims relate to preferred embodiments and refinements.

Accordingly, the invention relates to a device for the artificialweathering of samples, having a weathering chamber which containsholding means for holding samples to be weathered and a UV radiationarrangement for applying UV radiation to the samples. The invention isessentially characterized in that the UV radiation arrangement comprisesan arrangement of UV light-emitting diodes (LEDs). The inventiontherefore makes it possible to use of the availability of UVlight-emitting diodes which has occurred in recent years for aweathering tester, in particular ones which are based on GaN. GaN LEDscan now satisfactorily cover the entire UV range of the solar spectrum.The obtainable radiation densities are already so high that theradiation power of a conventional xenon lamp can readily be achieved bya multiple arrangement of UV LEDs.

The intended UV spectrum depends only on the band gap of thesemiconductor materials used in the LEDs. Additional undesired spectralcomponents, such as infrared spectral components, are therefore notproduced at all.

Another advantage is that the light intensity of the UV radiation whichis generated can be adjusted very easily by means of the currentdelivered to the LEDs. The emission spectrum can also be altered to alimited extent by adjusting the current.

Owing to the compact nature of the LEDs, they can be arranged in theform of LED arrays. By mounting them on flexible printed circuit boards,it is also possible for geometrically non-planar samples to be exposedalmost uniformly, or for a number of samples inside the weatheringchamber to be exposed uniformly. Virtually any irradiation surfaces canbe achieved since LED arrays are relatively easy to scale.

LEDs are furthermore known to have long lifetimes, of the order of 1500hours or more.

Spectral modification of such a UV radiation arrangement can be achievedby providing a plurality of different types of light-emitting diodeswith different spectral emission characteristics, in particular so as tosimulate the ultraviolet spectral component of natural solar radiation.This makes it possible to ensure that the UV-A component and the UV-Bcomponent of the solar spectrum are taken into account in a consistentlyrealistic way. Since the different types of light-emitting diodes canalso be operated individually, it is also possible to study the effectof exposing the material samples to individual wavelength ranges.

It is furthermore possible to provide at least one other type oflight-emitting diode in addition, the spectral emission of which lies inthe visible spectral range so that, in particular, part of the visiblespectral component of natural solar radiation can also be simulated. Ifdesired, essentially the entire spectrum of natural solar radiation canbe covered in this way by providing different types of LEDs withdifferent emission characteristics.

The LED arrangement is preferably provided as a regular arrangement ofthe LEDs, i.e. in particular in the form of a matrix of rows andcolumns.

It may be necessary to expose material sample which has a non-planarsurfaced topology. In this case, the LED arrangement may be designed andarranged so that the LEDs face the sample surface of the material samplewith an equal distance, so as to achieve uniform exposure of thematerial sample. If a plurality of material samples are being exposed,then it is equally well possible to provide the LEDs at equal distancesfrom the sample surfaces of the plurality of material samples. The LEDarrangement can accordingly be adapted to the profile or topology of thesingle sample surface, or of the plurality of sample surfaces.

In particular, it may be possible to adapt the LED arrangement bymounting the LEDs on flexible printed circuit board, in particular aso-called flexboard. The mounting may be carried out in a manner whichis known per se by using surface mount technology (SMT), with amultiplicity of LEDs being mounted on a printed circuit board (PCB). Itis then possible to use an LED design which, for example, is describedin the article “Siemens SMT TOPLED for surface mount technology” by F.Möllmer and G. Wait1 in the journal Siemens Components 29 (1991), volume4, page 147 in conjunction with Illustration 1. This form of LED isextremely compact and makes it possible to arrange a multiplicity ofsuch LEDs in a row or matrix arrangement.

If the LEDs are mounted on a flexible printed circuit board, then thismay be supported by fitting it to a holding body so that it adopts thesurface shape and topology of the latter. This holding body may consistof a thick metal plate, so that it simultaneously acts as a heat sink.The metal plate, or another support, may be pliable in a geometricallystable way so that it is possible to adapt to modified sample shapes.The holding body then needs to be fastened to an inner wall of theweathering chamber.

As an alternative to this, a flexible printed circuit board itself maybe designed, in terms of its thickness and its material, in such a waythat it is pliable and respectively maintains the new state in ageometrically stable way.

In a conventional weathering tester, the sample holding means are formedby a holding frame closed in a ring shape, which extends concentricallyaround the radiation source and, in particular, to which a rotationalmovement around the radiation source can be imparted. If the presentinvention is to be used in a conventional weathering tester, then theLED arrangement may be provided inside the ring-shaped holding frame,likewise as an arrangement closed in a ring shape. In particular, atubular holding body may be provided inside the holding frame andconcentrically with it, the LEDs being fastened in a desireddistribution on the outer circumference of the tubular holding body andelectrically connected in a suitable way. The LEDs are preferablymounted on a flexible printed circuit board which is in turn placed onthe outer circumference of the tubular holding body and fastened to it.The tubular holding body may be made of a metal, so that it constitutesa heat sink for dissipating heat from the LEDs.

The present invention will be explained in more detail below withreference to exemplary embodiments in conjunction with the figures ofthe drawing, in which:

FIG. 1 shows a longitudinal section of an exemplary embodiment of adevice according to the invention for the artificial weathering ofsamples;

FIG. 2 shows a cross section of the tubular holding body shown in FIG. 1with, fastened to it, the flexible printed circuit board which carriesthe light-emitting diodes;

FIG. 3 shows a flexible printed circuit board which is fastened to apliable support and carries the light-emitting diodes; and

FIG. 4 shows an exemplary embodiment of the use of three different UVLEDs and their spectral emission characteristics (solid lines) and thecumulative spectral emission curve (broken line).

FIG. 1 represents a longitudinal section of a device according to theinvention for the artificial weathering of samples.

A holding frame 2 closed in a ring shape is mounted so that it canrotate in a weathering chamber 1, and samples 3 or work-pieces can beheld on its inner wall. The holding frame 2 has, in particular, acircular cross section. A tubular holding body 4 is positioned insidethe holding frame 2 and concentrically with it, by fixing it to theupper wall of the weathering chamber 1. A flexible printed circuit board5 is placed around the outer circumference of the tubular holding body 4and is fastened to it in a suitable way. UV light-emitting diodes 6 aremounted on a regular arrangement on the flexible printed circuit board 5by using surface mount technology. These may comprise different types oflight-emitting diodes with different spectral emission characteristics.They may furthermore be electrically operated individually, and eachindividual light-emitting diode may be operated in a variable way as afunction of time. Also, light-emitting diodes of one spectral type maybe electrically operated together and light-emitting diodes of anotherspectral type may likewise be electrically operated together. The entirefield of light-emitting diodes may be divided into a number ofsub-fields, each sub-field containing at least one light-emitting diodeof each spectral type to be used.

The holding frame 2 is preferably mounted so that it can rotate in sucha way that the rotation axis coincides with the axis of the tubularholding body 4, so that the samples 3 move on a circular path around theindividual light-emitting diodes 6 and at an equal distance from them.

In a manner which is known per se, the weathering chamber 1 may alsohave other artificial weathering instruments, for example moisturegenerators or the like, although these do not play an essential part inthe present invention and will not therefore be discussed in detail. Forexample, an air flow may also be blown into the weathering chamber 1 andsweep past the samples 3 in a vertical direction.

FIG. 2 represents a cross section of the tubular holding body 4 inFIG. 1. The flexible printed circuit board 5 is form-fitted around theouter circumference of the tubular holding body 4. The UV light-emittingdiodes 6 are applied to it by using the SMT mounting technology which isknown per se. This will not be described in detail because it is knownin the prior art. The tubular holding body 4 may be made of a metal oranother material with good thermal conductivity, so that the heatproduced in the light-emitting diodes 6 can be dissipated efficiently.Optionally, the air flow produced in the weathering chamber 1 may alsobe passed through the interior of the tubular holding body 4 in order todissipate the heat from it.

As shown, the LEDs 6 are arranged in a matrix. Instead of this, the rowsmay also be arranged alternately with an offset between LEDs that lieabove one another, with one LED respectively being placed level with thegap between the two LEDs arranged in the row above.

The dynamic electrical operability of the individual light-emittingdiodes may, for example, may be used for energy-saving operation of theUV radiation arrangement. Specifically, for example, if only arelatively small number of material samples are to be artificiallyweathered, then they may be fastened next to one other on the holdingframe 2 over a particular limited angular sector of it. When the holdingframe 2 is set in rotation, then only those light-emitting diodes whichlie in the relevant angular sector are always supplied with current, sothat the material samples are exposed to the light-cone of UV radiationmoving around at an angular speed which is the same as the angular speedof the holding frame 2. It is merely necessary to provide suitableelectrical operation and programming of the light-emitting diodes 6.

As mentioned above, the entire diode field of the light-emitting diodes6 may be divided into sub-fields which respectively contain at least onelight-emitting diode of a particular spectral type. Spectrally differentlight-emitting diodes may be arranged in each of these sub-fields, sothat radiation approximately comparable to the solar spectrum can beemitted.

But if only the UV component of the solar spectrum is to beapproximately simulated, for example, then three differentlight-emitting diodes with three different emission curves in the UVspectrum may be used. This is represented in FIG. 4 where, by way ofexample, the emission curves of three spectrally differentlight-emitting diodes which add up to give an overall emission curve areplotted between the wavelengths 300 nm and 400 nm. Since the threelight-emitting diodes can be operated independently of one another, theemitted UV spectrum can therefore be adjusted flexibly.

The way in which a conventional weathering tester may be constructedaccording to the invention was described with reference to FIG. 1. Animportant point of the present invention goes beyond this, however,since it permits spatial adaptation of the UV radiation arrangement toone or more of the material samples to be exposed. This is shown in FIG.4 by way of example in relation to a material sample 3 which has aparticular surface topology. The invention now makes it possible toexpose this material sample 3 in such a way that the distance betweenthe light-emitting diodes 6 and the sample surface is spatiallyconstant. In order to do this, the light-emitting diodes 6 are fastenedon a flexible printed circuit board 5 in the manner already describedabove. Since in general the flexible printed circuit board 5 itself isnot geometrically stable, it is applied to a substrate 7 whose shape canbe altered and which keeps the shape when it is changed, that is to sayit is geometrically stable. The substrate 7 may, for example, be alightweight pliable metal sheet which again acts simultaneously as aheat sink for the heat to be dissipated from the light-emitting diodes6. The substrate 7 then merely needs to be fastened in a suitable way onthe inner wall of the weathering chamber. It is also conceivable to usea flexible printed circuit board which can be altered in a geometricallystable way owing to the way in which it is constructed, so that it isunnecessary to use an additional substrate 7.

If a rotational movement is imparted to the material sample 3 in FIG. 4,then the LED arrangement may likewise be moved in co-rotation with thesame angular speed, so that the material sample 3 and the LEDarrangement are always in a constant spatial relation to one another.

1. Device for the artificial weathering of samples, having a weatheringchamber (1) which contains holding means (2) for holding samples (3) tobe weather and a UV radiation arrangement for applying UV radiation tothe samples, characterized in that the UV radiation arrangementcomprises an arrangement of UV light-emitting diodes (LEDs) (6). 2.Device according to claim 1, characterized in that a plurality ofdifferent types of light-emitting diodes (6) with different spectralemission characteristics are provided so as to simulate the ultravioletspectral component of natural solar radiation.
 3. Device according toclaim 1 or 2, characterized in that at least one other type oflight-emitting diode is also provided, the spectral emission of whichlies in the visible spectral range so as to simulate part of the visiblespectral component of natural solar radiation.
 4. Device according toclaim 3, characterized in that the various types of LEDs (6) essentiallycover the entire spectrum of natural solar radiation.
 5. Deviceaccording to claim 1, characterized in that the LED arrangement isprovided as a regular arrangement of the LEDs (6).
 6. Device accordingto claim 1, characterized in that the LED arrangement is designed andarranged so that the LEDs (6) face the sample surface of a sample (3) orthe sample surfaces of a plurality of samples with an equal distance. 7.Device according to claim 6, characterized in that the LED arrangementis designed so that it can be adapted to the profile of the singlesample surface or of the plurality of sample surfaces.
 8. Deviceaccording to claim 1, characterized in that the LEDs (6) are mounted ona flexible printed circuit board (5).
 9. Device according to claim 8,characterized in that the flexible printed circuit board (5) is fastenedon a holding body (4; 7) which is designed as a heat sink.
 10. Deviceaccording to one of the preceding claims, characterized in that theholding means (2) are formed by a holding frame (2) closed in a ringshape, which extends concentrically around the radiation source and towhich a rotational movement around the radiation source can be imparted.11. Device according to claim 10, characterized in that the LEDarrangement is provided as an arrangement closed in a ring shape insidethe holding frame (2).
 12. Device according to claim 11, characterizedin that the LED arrangement or a flexible printed circuit board (5), onwhich the LEDs (6) are fastened, is fastened to a tubular holding body(4) which is designed as a heat sink.
 13. Device according to one of thepreceding claims, characterized in that the LEDs (6) can be operatedindividually as a function of time.
 14. Device according to one of thepreceding claims, characterized in that the LEDs (6) are made on thebasis of GaN.